Silicoaluminophospate (SAPO) is a material having a three-dimensional microporous crystal framework structure of PO2+, AlO2−, and SiO2 tetrahedral units, and whose essential empirical chemical composition in the as-synthesized form and on an anhydrous basis can be represented as follows:mR:(SixAlyPz)O2 wherein “R” represents at least one organic structure directing agent present in the intracrystalline pore system; “m” represents the moles of “R” present per mole of (SixAlyPz)O2; and “x,” “y,” and “z” represent respectively the mole fractions of silicon, aluminium, and phosphorus present in the oxide moiety.
Light olefins are traditionally produced from hydrocarbon feed stocks via thermal cracking of natural gas liquids or petroleum based naphtha and/or fluid catalytic cracking (FCC) of petroleum based feedstocks. With the increasing demand of light olefins, especially ethylene and propylene, alternate routes have been widely explored. Catalytic conversion of alcohols such as methanol to light olefins on molecular sieves is one of the most promising alternative routes to produce ethylene and propylene. This is especially true because methanol may be made from syngas derived from coal, methane, or biomass.
Catalytic conversion of methanol (and other light alcohols) to light olefins using microporous crystallite SAPO molecular sieves has been described by Kaiser (U.S. Pat. No. 4,499,327). The crystal structure, the silicon content and distribution, and the crystal size of the SAPO molecular sieves are among important features of the SAPO molecular sieves for maximizing the selectivity of catalytic conversion to light olefins.
There are a number of different structures of SAPOs which are represented by different framework types. These SAPOs include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO-41, and SAPO-56. Of these structures, SAPOs represented by framework type CHA (as described in Atlas of Zeolite Framework Types, 2007, 6th Edition, page 96) are known to be selective for the methanol-to-olefins (MTO) reaction (Kaiser, U.S. Pat. No. 4,499,327). In particular, SAPO-34, a CHA framework type with a pore opening of about 4 A and cylindrical cages within the structure of about 10×6.7 Å, is highly selective for the MTO reaction. However, the presence of other SAPOs such as SAPO-5 or SAPO-11 with SAPO-34 tends to produce undesired products (Stud. Surf. Sci. Catal., 61, 429 (1991). Hence, it is very important to produce SAPO-34 molecular sieves with high structural purity for the MTO reaction.
Furthermore, SAPO-34 molecular sieves with low silicon content and uniform distribution are important for maximizing the selectivity to light olefins in the MTO reaction (Microporous and Mesoporous Materials, 29, 117-126 (1999); Microporous and Mesoporous Materials 53, 97-108 (2002)). Small crystals of SAPO-34 molecular sieves are important to reduce undesired coke formation and improve lifetime of the catalyst (Microporous and Mesoporous Materials 29, 191-203 (1999)). Moreover, features such as flammability, boiling point, toxicity, and amount of the structure directing agent as well as filterability and yields of solid SAPOs recovered during the synthesis have important practical implications for commercial production of SAPO-34 molecular sieves.
During the synthesis of SAPOs, structure directing agents, which are also called templates, are typically used to direct the formation of particular types of framework structures. However, the structure directing agents' effect on the final crystalline structure of SAPOs varies. As a result, it is very difficult to produce relatively pure SAPO-34 structure using structure directing agents currently known to make SAPO-34. Lok et al describe the synthesis of SAPO-34 molecular sieves (along with other SAPO structures) with respect to various structure directing agents and synthesis conditions in U.S. Pat. No. 4,440,871. While certain structure directing agents direct or initiate formation of SAPO-34, other crystalline structures such as SAPO-5 are also formed during the synthesis.
Furthermore, those structure directing agents that are currently known to be more specific for making SAPO-34, such as tetraethylammonium hydroxide (TEAOH), diethylamine (DEA), triethylamine (TEA), or morpholine, have other practical implications. For example, Juan Tan et al discloses that TEA may be used to manufacture small crystal sizes of SAPO-34 (Microporous and Mesoporous Materials, 53 97-108, 2002). However, TEA is volatile, toxic, and relatively noxious, and therefore difficult to use in the commercial production of SAPO-34.
U.S. Pat. No. 4,677,243 discloses a method for synthesizing SAPO-34 using tetraethylammonium hydroxide (TEAOH) as a structure directing agent. While the major phase of the recovered crystalline product is SAPO-34, the product contains other structural impurities. Moreover, this method produces very small crystals of SAPO-34 (less than 1 micron), which are difficult to separate. In addition, TEAOH is also an expensive chemical which limits its practical use in the commercial production of SAPO-34.
US 2012/0203046 A1 also discloses a method for synthesizing SAPO-34 using two structure directing agents, TEAOH and DEA. However, no experimental data is disclosed regarding the structural purity of the solid product separated from the slurry comprising crystallized SAPO-34. Moreover, DEA is volatile, toxic, and relatively noxious, and therefore difficult to use in the commercial production of SAPO-34.
Furthermore, alkanolamines (also named aminoalcohols) either alone or in combination with other structure directing agents are disclosed as suitable to synthesize various types of SAPO frameworks. Alkanolamines have high boiling points, high flashpoints and are relatively less toxic. However, the disclosed synthesis methods using alkanolamines as structure directing agents do not produce SAPO-34 or produce SAPO-34 with low structural purity. For example, Chae et al disclose using N,N-diethanolamine to form SAPO-5, an AFI type of structure. Moreover, Chae et al disclose using triethylamine to form a mixture of SAPO-5 and SAPO-34 (Journal of Nanoscience and Nanotechnology, 10, 195-202, 2010). However, there is no mention of relative structural purity of SAPO-34.
U.S. Pat. No. 4,310,440 describes that ALPO-5, an analogue of SAPO-5, is prepared using triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N-methyldiethanolamine, and N-methylethanolamine as structure directing agents. However, there is no mention of synthesis of SAPO-34.
U.S. Pat. No. 6,767,858 discloses a method of synthesizing SAPO-34 using N-methylethanolamine as a structure directing agent at a temperature of 170° C. for 20 hours to 14 days with a SAPO-34 yield of 4.2%. The SAPO-34 yield increases to 27.1% when HPF6 is added as the fluorine source for the synthesis.
European patent application No. 0993867 discloses that SAPO-34 may be prepared using diethanolamine at 200° C. for 60 hours. However, no purity, yield, or physical properties are disclosed. It is also noted that this patent application discloses making SAPO-5 from the same components and the same method by just using different amounts of diethanolamine. In addition, there are no details provided on structural purity or yield of SAPO-5.
Therefore, as discussed above, structure directing agents currently known to form SAPO-34 have limited practical use due to the properties such as high toxicity, low boiling points, and low flashpoints (hence high pressures generated during synthesis). Other structure directing agents, such as alkanolamines which have high boiling points and high flashpoints and are relatively less toxic, do not yield SAPO-34 with high structural purity. Additionally, the methods described in literature do not yield small and highly uniform SAPO-34 crystals necessary for practical use.